Teacher Material

Using Video To Quantify a Reflex Response to a Stimulus

Craig Patterson

Overview

The purpose of this lab is to provide an interesting and unique approach to measuring reflexes. Students use video equipment to measure the response of the patellar reflex to various strengths of stimuli. By counting the frames of video required for the reflex hammer to reach the knee from a known distance, students estimate force of the stimulus. The response is measured by how far the foot moves measured by a grid. Students quantify the force, the response of the reflex, and graph the results. Students gain an understanding of structure of the reflex, motor units, and stretch receptors. They also learn that there are many stretch receptors and motor units that make up a reflex and that they work independently. More stretch receptors are stimulated as the force increases.

Biological Concepts

The purpose of this lab is to demonstrate to students the effects of the strength of a stimulus on a patellar reflex. Students will observe that the stimulus required to illicit a reflex depends on the amount of the force of the stimulus. They will also see the response of the reflex depends on the force of the stimulus. The demonstration shows that a certain amount of force is required to stimulate stretch receptors initiating the knee-jerk reflex and that an increase of force will stimulate more stretch receptors causing a stronger reflex action.

Students will also practice measuring a relative force and response by the knee jerk reflex using video tape as a tool for analysis.

Class Time

The time required for this activity is approximately 40 - 50 minutes.

Background Information

Students should have a basic knowledge of how a reflex arc works, understand the concept of motor unit, and stretch receptors. An appropriate conclusion to this activity is to understand that there are many stretch receptors making up a reflex.

Materials

•

• Video Camera and Tape

• Video Player With Individual Frame Advance

• Tripod (Optional if there are three/group)

• Meter Stick

• Reflex Hammer

• Poster Board

• Black Magic Marker

Teacher Preparation

Gather the video taping and video playing equipment. Demonstrate to students the setup explained and illustrated below (Figure 1). All of the analysis is done using video tape.

1. Construct a 5 cm square grid on the poster board using dark lines so that they will easily be seen on the video tape.

2. Select a student to sit on a counter top with his/her legs hanging for a knee-jerk reflex test.

3. Position the camera approximately eight feet from the student making sure his/her torso and legs are in the camera’s frame.

4. Place the grid between the subject’s legs and support it using clamps on a ring-stand.

5. Have the subject hold the meter stick perpendicular to his/her knee.

6. A second student stands behind the grid on the opposite side of the camera and measures a specific distance from the knee ( I recommend at least 20 cm).

7. The second student then slides the reflex hammer along the meter stick until it strikes the area below the knee. Make sure that the arm and wrist stay in line (parallel) with the handle of the reflex hammer. This is done to reduce the error from any twisting motion that would increase torque and velocity.

8. After the students attempt several trials of slow, medium, and fast strikes using the reflex hammer, they video tape the procedure striking the reflex area at least 20 times and varying the speed of the strikes. Equal attempts at striking should be made at relatively slow speeds (slow speeds should not result in a reflex) to relatively fast speeds.

9. Students view their strikes using the video tape player’s single frame advance and counting the number of frames required to move the hammer to the knee. Each frame is 33 ms in length and this value could be quantified as a time or the number of frames can be used as a relative measurement. This data is recorded in a data table.

10. Students also measure the movement of the foot. There are several approaches to make this measurement. One suggestion is to count how high the foot travels using the toe as a reference. Other measurement strategies may use complete range of movement or speed, etc.

11. The data are graphed.

Figure 1. Illustration of student sitting on the desk and the second student sliding the reflex hammer along the meter stick. The view is looking at the grid in the same direction as the camera.

The sample data set included here consists of only twelve strikes, four samples showing slow speeds, four moderate speeds, and four higher speeds. Speed is reflected by the number of frames it took for the reflex hammer to strike. The samples occur in clumps and were not continuous.

Extensions/Variations

An extension of this activity would be to have the student hold his/her leg up slightly to increase the tension on the reflex and repeat the trials. What students should see is a response using much less force because the stretch receptors are much tighter and need less stimulation to respond.

Resources

Tortora, G.J. and S.R. Grabowski. Principles of Anatomy and Physiology, Eighth Edition. Harper Collins College Publishers, 1996. Pages 371-374.

About The Author

Craig Patterson is a biology teacher at Evanston High School in Evanston, WY. Craig can be contacted at Evanston High School, PO Box 6002, Evanston, WY 82930

Student Activity

NAME: ____________________________________ DATE: _________ PERIOD: ________

Using Video To Quantify a Reflex Response to a Stimulus

1. Create a table to record your measurements in your notebook. Be sure to include these three variables: distance to knee, number of frames or ms elapsed, and distance that the foot moved.

2. Create a graph of your data. Be sure you plot the data correctly, labeling the dependent and independent variables correctly.

3. Base on your knowledge of the reflex arc, explain why you obtained the results that your did. Be sure to include a discussion of the how stretch receptors and motor units are involved creating the stretch reflex.

4. Describe what would happen if you raised the leg so that there was additional tension on the knee and repeated the same experiment. What would happen if you relaxed the knee even more?

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